1. Field of the Invention
The present invention relates to a liquid-crystal display (LCD) device. More particularly, the invention relates to a method of fabricating a substrate for a LCD device having a photosensitive organic material layer as an insulating layer, a method of a LCD device using a photosensitive organic material layer formed on one of two substrates, and a semi-transmissive type LCD device.
2. Description of the Related Art
LCD devices have an advantage of compactness, thinness, and low-power dissipation. Therefore, in recent years, LCD devices have been practically applied to various fields such as OA (Office Automation) and portable electronic instruments fields. As known well, LCD devices are usually classified into the “transmissive type” and the “reflective type”.
With the transmissive type LCD device, “an illumination source (i.e., a backlight)” is incorporated. This is because the device of this type has no function of emitting light unlike CRTs (Cathode-Ray Tubes) and EL (ElectroLuminescent) devices. The light emitted from the backlight is controlled (i.e., transmitted or blocked) with the liquid-crystal panel, thereby displaying images on the screen. Thus, the transmissive type LCD device is capable of displaying bright images regardless of ambient conditions. However, the backlight dissipates comparative much power and occupies almost half of the total power dissipation of the device. Therefore, the backlight forms the cause of power dissipation increase. In particular, when the device is driven with a battery, the maximum operable time of the device will be largely shortened due to the backlight. To avoid the disadvantage of shortened operable time, a large-sized battery can be incorporated. In this case, however, the total weight and size of the device will increase, degrading the advantage of compactness and lightweight.
To solve the problem of large power dissipation due to the backlight, the “reflective type” LCD device has been developed, where images are displayed on its screen utilizing ambient light. With the reflective type LCD device, a reflection layer or plate is provided instead of the backlight. The ambient light reflected by the reflection layer/plate is controlled (i.e., transmitted or blocked) with the liquid-crystal panel, thereby displaying images on the screen. In this way, the backlight is unnecessary and therefore, the reflective type LCD device has an advantage that the power dissipation is conspicuously lowered and that the compactness and lightweight is improved. Instead, the device of this type has a disadvantage that visibility will degrade under the condition that usable ambient light is insufficient (i.e., where the device is used in the dusk or ill-lighted places).
Accordingly, to solve both the problem of large power dissipation due to the backlight and the problem of the possibility of visibility degradation dependent on the ambient situation, the “semi-transmissive type” LCD device has been developed. This device is capable of displaying images as the transmissive type device or the reflective type device according to the necessity, where each pixel area has a transmission region and a reflection region. The reflection layer/plate is provided only in the reflection region. No reflection layer/plate exists in the transmission region. If the ambient light is sufficient, the backlight is turned off and the device is operated as the reflective type device, thereby reducing the power dissipation. On the other hand, if the ambient light is insufficient, the backlight is turned on and the device is operated as the transmissive type device, thereby raising the visibility.
Conventionally, with the above-described prior-art reflective and semi-transmissive type LCD devices, to improve the reflection characteristic or performance of the reflection regions, a patterned photosensitive organic material layer is used. Specifically, a photosensitive organic material layer with a thickness of several micrometers is formed on a substrate by coating. Thereafter, the photosensitive organic material layer is subjected to an exposure process and a development process, thereby patterning the layer and forming desired projections and depressions on its surface. Subsequently, a reflection layer is selectively formed on the projected and depressed surface of the photosensitive organic material layer. Thus, ambient light will be irregularly reflected by the reflection layer as desired, improving the reflection characteristics.
FIGS. 1A to 1D show a method of fabricating the TFT (Thin-Film Transistor) substrate of a prior-art reflective type LCD device, which is disclosed in the Japanese Non-Examined Patent Publication No. 2000-250025 published on Sep. 14, 2000. Although many pixel areas are arranged in a matrix array, only two pixel areas are shown in FIGS. 1A to 1D for simplification of description. This method is termed the “1 PR process”.
First, as shown in FIG. 1A, gate electrodes 102 and common electrode lines 102a are formed on a transparent dielectric plate 101. A common gate dielectric layer 103 is formed on the plate 101 to cover the gate electrodes 102 and the common electrode lines 102a. Patterned (i.e., island-shaped) amorphous silicon (a-Si) layers 104a, patterned (i.e., island-shaped) n+-type a-Si layers 104b, source electrodes 105, and drain electrodes 106 are formed on or over the gate dielectric layer 103. Thus, TFTs 107 for the respective pixel areas are formed on the substrate 101. A passivation layer 108 is formed to cover the TFTs 7. The state at this stage is shown in FIG. 1A. The reference numeral 121 denotes channel regions of the respective TFTs 107.
Subsequently, a base surface having projections and depressions is formed to make inequalities of a reflection electrode layer. Concretely, as shown in FIG. 1B, a photosensitive organic resin is coated onto the passivation layer 108, forming a photosensitive organic resin layer 110 as an insulating layer. Thereafter, using different masks, the contact-hole areas are exposed to ultraviolet (UV) light at a first exposure value UV1 and the inequality areas are exposed to UV light at a second exposure value UV2. The second exposure value UV2 for the inequality areas is set at 10 to 50% of the first exposure value UV1 for the contact-hole areas. The state at this state is shown in FIG. 1B.
Next, the photosensitive organic resin layer 110 thus exposed is developed. In this development process, a fact that the dissolution rate of a positive-type photosensitive organic resin varies largely dependent on the decomposition rate of the sensitizer contained in the resin is utilized. Specifically, due to the difference between the first and second exposure values UV1 and UV2, the decomposition rate of the sensitizer of the layer 110 in the inequality areas and that in the contact-hole areas are different from each other. As a result, the dissolution rate in the inequality areas is different from that in the contact-hole areas. The development time is set in such a way that the parts of the layer 110 in the contact-hole areas are fully resolvable. In this way, contact holes 111 with a depth A′ are formed to penetrate the layer 110 and at the same time, surface depressions 113 with a depth B′ are formed on the layer 110, as shown in FIG. 1C.
The above-described exposure process may be carried out using a halftone mask, where the mask has reflection parts selectively formed for the projections 114 of the photosensitive resin layer 110, transmission parts selectively formed for the contact holes 111 (and the G-D conversion sections and terminal sections, not shown), and semi-transmission parts selectively formed for the depressions 113 of the layer 110. By the use of the halftone mask, there is an advantage that the projections 114 and depressions 113 of the layer 110 and the contact holes 111 are simultaneously formed through a single exposure process.
Needless to say, the above-described exposure process may be carried out using an ordinary mask having only reflection parts and transmission parts. In this case, the inequality areas and the contact-hole areas of the photosensitive organic resin layer 110 are subjected to separate exposure processes with different exposure values. Additionally, in the process of forming the base surface with projections and depressions for a reflection electrode layer, the layer 110 may have a single-layer or two-layer structure.
Finally, as shown in FIG. 1D, an aluminum (Al) layer is deposited on the photosensitive organic resin layer 110 thus inequalized over the whole plate 101 by a sputtering or evaporation process, thereby forming a reflection electrode layer 112 on the layer 110. Thereafter, although not shown, an alignment layer made of polyimide is formed on the reflection electrode layer 112. As a result, the TFT substrate is completed.
The TFT substrate formed is then coupled with an opposite substrate (not shown) having a color filter, a black matrix, an opposite electrode, and an alignment layer in such a way as to sandwich a liquid-crystal layer. Thus, a reflective type LCD device is completed.
Next, a prior-art transmissive type LCD device is explained. FIG. 2 shows the structure of a prior-art LCD device of this type.
As shown in FIG. 2, this device comprises an active matrix substrate 213 on which switching elements (i.e., TFTs 207) are formed, an opposite substrate 217 on which a color filter 215 and a black matrix (not shown) are formed, a liquid-crystal layer 218 sandwiched by the substrates 213 and 217, and a backlight 220 located behind the active-matrix substrate 213. Although many pixel areas are arranged in a matrix array, only one pixel area is shown in FIG. 2 for simplification of description.
On the active-matrix substrate 213, gate or scanning lines (not shown), data or signal lines (not shown), TFTs 207, and pixel electrodes (not shown) are formed over a transparent, dielectric plate 201. The TFTs 207, each of which comprises a gate electrode 202, an island-shaped a-Si layer 204, a source electrode 205, and a drain electrode 206, are arranged near the respective intersections of the gate and data lines. The drain electrodes 206 of the TFTs 207 are connected to the corresponding data lines. The source electrodes 205 of the TFTs 207 are connected to the corresponding pixel electrodes. The gate electrodes 202 of the TFTs 207 are formed on the plate 201. A common gate dielectric layer 203 is formed on the plate 201 to cover the gate electrodes 202. A passivation layer 208 is formed on the layer 203 to cover the TFTs 207. A transparent electrode layer 209, which is made of ITO (Indium Tin Oxide), is formed on the passivation layer 208 to be connected to the source electrodes 205.
Each of the pixel areas is divided into a reflection region 222a that reflects ambient light and a transmission region 222b that allows the light from the backlight 220 to penetrate through the region 222b. 
In the reflection region 222a, a photosensitive organic resin layer 210 is selectively formed on the electrode layer 209. The layer 210 has inequalities, i.e., projections and depressions. A reflection electrode layer 212, which is made of Al or an alloy of Al, is selectively formed on the layer 210. The reference numeral 211 denotes the contact hole penetrating through the photosensitive organic resin layer 210. An alignment layer 219a is formed on the layer 210 to cover the reflection electrode layer 212. On the other hand, in the transmission region 222b, the photosensitive organic resin layer 210 does not exist and the alignment layer 219a is formed directly on the electrode layer 209.
On the opposite substrate 217, the color filter 215 and the opposite electrode 216 are successively formed on a transparent, dielectric plate 214. An alignment layer 219b is formed to cover the electrode 216.
With the prior-art semi-transmissive type LCD device shown in FIG. 2, the light from the backlight 220 by way of the active-matrix substrate 213 transmits the liquid-crystal layer 218 in the transmission region 222b of the pixel area and then, goes out of the opposite substrate 217. In the reflection region 222a of the pixel area, ambient light which has entered the liquid-crystal layer 218 through the opposite substrate 217 is reflected toward the opposite substrate 217 by the reflection electrode layer 212. Thereafter, the ambient light thus reflected penetrates through the substrate 217 again and goes out of the same.
The thickness of the photosensitive organic resin layer 210 is set in such a way that the gap or thickness of the liquid-crystal layer 218 in the reflection region 222a is approximately equal to half of the gap or thickness of the layer 218 in the transmission region 222b. Therefore, the optical path length of the layer 218 in the reflection region 222a is approximately equalized to that in the transmission region 222b, thereby adjusting or controlling the polarization state of the output light.
The method of fabricating the prior-art semi-transmissive type device of FIG. 2 is approximately the same as the method for the prior-art reflective type device of FIGS. 1A to 1D, except that all the part of the photosensitive organic resin layer 210 existing in the transmission region 222b is removed in the step of forming the projections and depressions of the layer 210.
The above-described prior-art methods of the LCD devices have the following problems.
With the method of fabricating the prior-art reflective type device of FIGS. 1A to 1D, the contact-hole areas are exposed to UV light at the first exposure value UV1 and the inequality areas are exposed to UV light at the second exposure value UV2 different from the first exposure value UV1. Thus, the photosensitive resin layer 110 is selectively removed in the contact-hole areas to form the contact holes 111. On the other hand, the layer 110 is selectively removed in the inequality areas to form the inequalities of the layer 110.
Moreover, with the method of fabricating the prior-art semi-transmissive type device of FIG. 2, like the device of FIGS. 1A to 1D, the reflection regions 222a and the transmission regions 222b are exposed to light at different exposure values, respectively. Thus, the photosensitive resin layer 210 is selectively removed in the reflection regions 222a to form the contact holes 211 and the inequalities of the layer 210. On the other hand, the layer 210 is entirely removed in the transmission regions 222b. 
As shown in FIG. 3, the prior-art semi-transmissive type device of FIG. 2 has the rectangular display section 222 for displaying images and the rectangular-ring-shaped terminal section 223 for interconnection with the external circuitry. The terminal section 223 is formed outside the display section 22 in such a way as to surround the entire section 222. The photosensitive resin layer 210 existing in the terminal section 223 needs to be removed in the process of forming the inequalities of the layer 210 and the contact holes 211. In the method of fabricating the prior-art device of FIG. 2, the exposure value for removing the layer 210 in the terminal section 223 is equalized to the exposure value for selectively removing the layer 210 to form the contact holes 211 in the reflection regions 222a and that for removing the layer 210 in the transmission regions 222b. 
However, the device has inequalities not only caused by the TFTs 207 and the gate and data lines but also caused by the existence and absence of the gate dielectric layer 203 and the passivation layer 208. Therefore, when the photosensitive organic resin layer 210 is formed by uniformly coating, the thickness of the layer 210 varies according to these inequalities. The thickness of the layer 210 in the terminal section 223 is likely to be larger than that in the display section 222. This is because the gate dielectric layer 203 and the passivation layer 208 do not exist in the terminal section 223.
For example, the thickness of the photosensitive organic resin layer 210 in the contact-hole areas of the display section 222 is approximately 2 μm while the thickness of the layer 210 in the terminal section 223 is approximately 2.75 μm. To simultaneously remove the layer 210 in the contact-hole areas of the display section 222 and the terminal section 223, the exposure condition needs to be determined in such a way that the relatively thicker layer 210 in the terminal section 223 is fully removed. Therefore, the relatively thinner layer 210 in the display section 222 will be excessively exposed. Specifically, the layer 210 in the contact-hole areas of the reflection regions 222a of the display section 222 and the transmission regions 222b thereof will be in over-exposure state. If the display section 222 is over-exposed, some defects (e.g., stage image transfer and mask image reflection) will be observed in the displaying operation.
With a typical exposure apparatus, chucks and/or pins are provided on the exposure stage to hold the active-matrix substrate 213 thereon. The surface of the stage has a reflection factor or reflectance different from that of the chucks and/or pins. Therefore, if the photosensitive organic resin layer 210 of the substrate 213 is over-exposed, the exposing light penetrates through the substrate 213 and reflected by the stage surface and the chucks and pins, thereby affecting the layer 210. Due to this effect, the dissolution rate of the layer 210 will deviate from its desired values. As a result, an image of the exposure stage (i.e., the stage surface and the chucks and/or pins) will be slightly or thinly transferred to the layer 210 after the development process is completed. This phenomenon is termed the “stage image transfer”.
Moreover, with a typical exposure apparatus called the “stepper”, the exposure mask comprises a plurality of exposing patterns. The exposing operation (i.e., the shot) is carried out while one of the patterns is selected and the remainder is covered with the blind. In this case, the exposing light is likely to be reflected by the patterns to be covered and/or by the edges of the blind. The light reflected may be further reflected by a lens system incorporated in the exposure apparatus, being irradiated to the substrate 213 held on the stage as stray light. The stray light will affect the photosensitive organic resin layer 210 and as a result, the dissolution rate of the layer 210 will deviate from its desired values. Thus, an image of the undesired patterns on the mask and/or the blind edges will be slightly transferred to the layer 210 after the development process is completed. This phenomenon is termed the “mask image reflection”.